Projects

Ever since the invention of transistor in 1947, impurities in semiconductors have played a crucial role in microelectronics industry. By the end of the twentieth century, as the transistor size shrunk from micrometer range to nanometer regime, scientists and engineers have been increasingly interested in the understanding of physics of devices with countable discrete impurity atoms. Going beyond classical regime of transistor operation, a radical new approach is quantum computing, where the fundamental building block of devices is a qubit stored in nuclear or electron spin of a single impurity atom. Such devices promise to harness the power of the quantum nature of materials for the development of superfast machines capable of solving currently intractable problems, which are inaccessible with conventional computers.

We study group V impurities (such as P, As, Bi) in silicon which are promising for spin qubit devices and quantum computing architectures due to the associated long coherence times. Understanding quantum logic operations to be performed on
donor based silicon devices has required the construction of an effective spin
dynamics model coupled to, and informed by, empirical-based atomistic models. We have established a comprehensive atomistic tight-binding model to investigate the donor physics in silicon. This theoretical framework has been successful in the determination of exact
positions of phosphorous dopants in silicon after fabrication and over-growth
processes based on STM imaging data obtained by UNSW researchers. This result is important in the
design and optimisation of highly precise quantum logic gates for scalable quantum
computer architecture. Our team applied the
atomically precise spatial metrology technique to the STM images of correlated wave
functions measured for phosphorous dopant pairs. Based on a
systematic analysis of the symmetries of images, supported by a quantitative
comparison between experiment and multi-million-atom theory, we identified
the exact depths and in-plane locations of the two phosphorous atoms in the
measured pairs. This high-precision spatial metrology of dopant pairs allowed a
quantitative understanding of the valley interference processes and their
relation with the exchange interaction energies as a function of phosphorous
pair separations, confirmed through a direct comparison with the STM
measurements.

Selected Publications:

M. Usman et al, Nanoscale 9, 17013, (2017)

M. Usman et al, Nature Nanotechnology 11, 763, (2016)

M. Usman et al, (Invited Article) JPCM 27, 154207, (2015)

Bismide Alloys and Heterostructures:

Targets Applications:Photonic Devices, Photovoltaics, Spintronics

Designing new materials with engineered band-structure
properties is a topic of intense research interest in material science and
condensed-matter physics communities. While traditionally Arsenides,
Phosphides, and Nitrides have been the focus of research for photonic and
optoelectronic devices, recently a new class of materials known as Bismides has
emerged as promising medium for the design of devices. Bismides, which are
typically formed by replacing a small fraction of As atoms in GaAs or InAs with
Bi atoms, offer unique properties at the band-structure level which can be
exploited to overcome a number of challenges present in today's devices. For
example, Auger loss mechanism that severely degrades the efficiency of today's
InP-based devices is expected to be suppressed in Bismide based devices due to crossover
between band gap and spin split-off energies. A large tuning of the band gap
energy as a function of Bi fraction of alloy offer opportunities for targeting
wavelengths in telecommunication and infrared range. Other potential applications
for Bismide alloys are in the field of photovoltaics and thermoelectric
devices.

We have developed a comprehensive atomistic tight-binding framework to
investigate the electronic and optical properties of Bismide alloys and quantum
well. Our results have shown that by increasing Bi fraction above 10-11%, band
gap energy reduces below spin split-off energy, a proof-of-concept for
Auger-loss free photonic devices. Atomistic resolution studies have predicted a
crucial role of alloy disorder related effects, with important implications
towards understanding device characteristics and designing future devices with
tailored functionalities.

Self-assembled In(Ga)As/GaAs quantum
dots are a promising solid-state system, and are widely employed for the design of a variety of
optoelectronic devices and quantum information applications. Based on multi-million-atom simulations, we provide an in-depth understanding of their electronic and optical properties, and perform engineering of related geometry parameters for implementation of devices with tailored functionalities. We have investigated both single quantum dots, as well as large stacks of strongly-coupled quantum dots.